III. NEW CIRCUIT BREAKER TECHNOLOGIES
The concept of a solid state circuit breaker is not new. Numerous attempts have been made at implementing power semiconductor-based breakers, but few have made a significant positive impact on power systems. A fundamental
problem with most prior concepts is the desire to have the solid state breaker behave like a traditional electromechanical breaker in terms of fault let-through current and coordination. High let-through current enables traditional coordination with downstream breakers or fuses, but it does not complement the capability of modern power semiconductors. AC breakers
have been constructed using SCRs connected in inverse parallel as shown in Fig. 7. While offering silent operation and long life, these breakers still have ½ line cycle type response, do not current limit, and are not suitable for DC systems. Hence they provide minimal performance improvements to the overall power distribution system. For worst case asymmetrical fault currents (i.e. 200kA peak for 85kArms available current) fuses must be employed to protect the SCRs.
Thermal losses are significant due to the minimum SCR onstate voltage drop. A hybrid breaker (contact across the SCRs) can eliminate the conduction loss and need for an arcing contact structure (i.e. virtual arcless switching), but provides minimal system performance advantages.
Fig. 7. SCR based AC circuit breaker A superior solution is one in which breakers are current limiting by responding early and quickly to an increasing fault current. Current liming in a traditional system can present
problems with fault coordination. Simply adding a few current limiting breakers to a conventional system may be problematic. The entire system must be designed with current limiting breakers as key driver of the protection requirements. The Navy, with its “closed” shipboard power system, is in a
unique position to be able to take advantage of new technologies by defining a power system that takes advantage of new breaker technologies.
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